At the very small separations currently employed between the transducing heads and disks of a disk file, which may be of the order of 0.5.times.10.sup.-6 m, it is essential to enclose at least the head/disk assembly (HDA) completely and to provide within the enclosure a filter for filtering particulate and other contamination. Failure to remove particles of sizes comparable to the head/disk separation can lead to head crashes and damage to the recording medium.
Some typical filtration systems for earlier magnetic disk files are described in U.S. Pat. Nos. 3,489,800; 3,710,357; and 4,054,931. In the systems shown in these patents, fan blades, assisted to some extent by the rotation of the disks, were employed to produce a pressure differential to circulate air through a filter within the enclosure.
A more recent development in the design of disk file filtration systems has been the concept of the internally vented hub. By venting the hub assembly on which a stack of disks is mounted and slotting the spacers between the disks, the pressure differential created between the low pressure hub region and the high pressure region at the outer periphery of the disks is maximised causing a circulatory airflow around and between the disks. The filter must be located within this circulatory airflow. In such systems, scavenging of particles from between the disks is particularly effective and the pressure differentials obtainable are such that additional fan blades are usually not needed.
One example of this latter type of system is shown in U.S. Pat. No. 4,633,349. The disk file described in this application has a stack of disks separated by slotted spacers and mounted on a bell-shaped hub assembly. The disk stack rests on an outer flange of the hub assembly which includes a deeply dished spring clamp to hold the stack in place. This clamp has openings in it to permit communication between the inter-disk spaces and an axial region at one end of the hub assembly. The filter employed is a cartridge in the form of an annular plenum encircling the top of the hub assembly above the extreme disk of the stack. Air enters the filter through peripheral openings in the plenum. Within the plenum is a double layer of filter paper, the lower layer being annular and sealed around a central outlet hole located above and around the vented hub assembly. Air entering the plenum can only exit after passing through one or both of the filter papers from where it is drawn into the low pressure region of the hub assembly. However, the clearance gap between the filter outlet and the hub assembly constitutes a leakage path by which air can bypass the filer assembly.
In the prior art, as has been mentioned above, the pumping action causing the pressure differential to drive air through a filter is produced by rotation of a stack of disks, sometimes assisted by fan blades. A disk file having only a single disk yet no fan is shown in U.S. Pat. No. 4,268,878. To achieve an adequate pumping action, a stationary `control plate` is positioned over and close to the single disk. The plate has an annular gap around the disk hub. Rotation of the disk causes an outward pumping action in the region between the control plate and the disk. This draws air through the annular gap from an upper chamber above the control plate and pumps it outwardly. A return path to the upper chamber is provided via a filter in one corner of the enclosure.